Mobile communication systems were developed to provide mobile users with communication services. With the rapid advance of technologies, the mobile communication systems have evolved to the level capable of providing high speed data communication service beyond the early voice-oriented services.
Recently, standardization for a Long Term Evolution (LTE) system, as one of the next-generation mobile communication systems, is underway in the 3rd Generation Partnership Project (3GPP). LTE is a technology for realizing high-speed packet-based communications with the data rate of up to 100 Mbps, which is higher than the currently available data rate, and its standardization is almost complete.
In line with the completion of the LTE standardization, an LTE-Advanced (LTE-A) system is now under discussion, which improves a transfer rate by combining the LTE communication system with several new technologies. One of such technologies is Carrier Aggregation. The Carrier Aggregation is a technology allowing a terminal to use multiple downlink carriers and multiple uplink carriers unlike the conventional technology of using one downlink carrier and one uplink carrier for data communication.
Currently, the LTE-A is featured with the intra-eNB carrier aggregation only. This restricts applicability of the carrier aggregation function so as to a problem of failing aggregation of macro and pico or femto cells in a scenario where a plurality of pico or femto cells and a macro cell operate in an overlapped manner.